Assimilation of xylem-transported 13C-labelled CO2 in leaves and branches of sycamore (Platanus occidentalis L.)

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Abstract
Previous reports have shown that CO2 dissolved in xylem sap in tree stems can move upward in the transpiration stream. To determine the fate of this dissolved CO2, the internal transport of respired CO2 at high concentration from the bole of the tree was simulated by allowing detached young branches of sycamore (Platanus occidentalis L.) to transpire water enriched with a known quantity of 13CO2 in sunlight. Simultaneously, leaf net photosynthesis and CO2 efflux from woody tissue were measured. Branch and leaf tissues were subsequently analysed for 13C content to determine the quantity of transported 13CO2 label that was fixed. Treatment branches assimilated an average of 35% (SE=2.4) of the 13CO2 label taken up in the treatment water. The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles. Overall, the fixation of internally transported 13CO2 label by woody tissues averaged 6% of the assimilation of CO2 from the atmosphere by the leaves. Woody tissue assimilation rates calculated from measurements of 13C differed from rates calculated from measurements of CO2 efflux in the lower branch but not in the upper branch. The results of this study showed unequivocally that CO2 transported in xylem sap can be fixed in photosynthetic cells in the leaves and branches of sycamore trees and provided evidence that recycling of xylem-transported CO2 may be an important means by which trees reduce the carbon cost of respiration.

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